Cassini probe failed to 'taste' moon's geysers in flyby

This is the highest resolution view yet obtained of Enceladus’s north polar region. Compared to much of the moon’s southern hemisphere – and the south polar region in particular – the north polar region is much older and covered with craters. The craters are captured at different stages of disruption and alteration by tectonic activity and probably past heating from below

(Image: NASA/JPL/Space Science Institute)

The Cassini spacecraft has survived its passage though the ice plume of Enceladus and sent back close-up images and other data from this mysterious moon of Saturn, but not everything went according to plan. One vital experiment, which scientists hoped would help reveal the origin of the plume, failed to collect any data at the crucial moment.

Flybys of the moon planned for later in 2008 may be able to repeat the plume fly-through to try to collect the observations missed in this attempt.

As Cassini flew over the small moon on 12 March, passing only 200 kilometres from the base of the plume, an “unexplained software hiccup” prevented the spacecraft’s Cosmic Dust Analyzer (CDA) from transmitting data to the onboard computer.

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New software, designed to improve the ability of CDA to count particle hits, may be to blame. “We don’t know why it did not work,” says the instrument’s principal investigator, Ralf Srama of the Max Planck Institute of Nuclear Physics in Heidelberg, Germany. “We prepared very carefully.”

Among other things, CDA would have been looking for mineral grains, which might act as nucleation points for the ice crystals and could reveal whether the rocky core of Enceladus is connected with whatever drives the geysers.

Old and cratered

Other instruments on board were working fine, however. Cassini’s cameras captured pictures of the north pole of Enceladus. This northern terrain is old and cratered, unlike the fresh, young, smooth terrain of the south. That may be because the southern regions are more heavily blanketed by ice from the plume, which is blasted out from fissures near the south pole called “tiger stripes“.

Another instrument, the Composite Infrared Spectrometer (CIRS), got a close-up look at the tiger stripes, although some analysis is still needed before any images are released. “We’re looking for any kind of changes from the last flyby in 2005,” says Neil Bowles of Oxford University, UK, a member of the CIRS team.

If the heat source is powerful enough to melt a watery sea or ocean under Enceladus’s ice crust, then electrical currents in that sea could affect magnetic fields near the moon.

During the 12 March flyby, Cassini’s magnetometer found that the field of Saturn is bent around the plume. According to magnetometer team leader Michele Dougherty of Imperial College, London, UK, the new data will let them model the magnetic effects of Enceladus much better than before; but it’s not yet clear whether they will be able to tease out the small effect of an ocean.

Cassini will return in August, perhaps passing even closer to the source of the plumes, and again in October, when the dust analyser should get another chance to see exactly what is coming out of Enceladus.

Cassini&colon; Mission to Saturn – Learn more in our continually updated special report.